Bar code scanners produce a beam of light, often from a laser, that is appropriately shaped by beam forming optics and then scanned across a bar code symbol by a deflector such as an oscillating mirror or rotating polygon having mirror facets. Scattered light from the bar code is collected in collection optics and is incident on a photodetector in the scanner. The photodetector converts the return light into a time varying analog signal that is an electrical representation of the physical bar and space widths. Subsequent circuits convert this signal into a logic level pattern with analog timing that represents the bar code. This logic level pattern is sent to a computer and decoded to determine the characters in the message represented by the bar code. The bars and spaces are resolved by the laser spot where the scanning beam is incident on the code. Aperture stops and lenses have been used to shape the beam and the resolving spot.
Bar code scanners have used gas (HeNe lasers) that produce circular scanning spots. However, bar code symbols are often printed with low resolution printing processes, for example, using dot matrix printers, and may be subject to harsh environments or abrasion during handling. These factors cause voids to be present in the printed bars. Dust and dirt on the bars can result in obscuration of the spaces between the bars. The presence of these defects may cause reading errors.
It has been proposed to use an elongated scanning spot to decrease the probability of a reading error. The elongated, for example, elliptically shaped spot, with its major or long axis arranged parallel to the long dimension of the bars and spaces in the code, averages over the length of the bars and spaces and minimizes the effects of small defects. Anamorphic optics have been suggested for use in a bar code scanner with a gas laser which produces a circular spot in order to provide an elliptical laser spot (see, U.S. Pat. No. 4,721,860, issued Jan. 26, 1988).
Laser diodes have been used in scanners which, because their small size, as opposed to the gas laser, enables the scanner to be miniaturized. However, the beam from a laser diode is not symmetrical. For example, a visible laser diode, which is now commercially available from Toshiba (their model TOLD 9200), diverges approximately 34.degree. in one direction and 7.degree. in a direction orthogonal thereto. It has been proposed also, to use lenses and apertures to shape such a beam into a elliptical beam. However, such beams do not stay in the same width to height aspect ratio relationship. Rather, the beam flips so that its long dimension, initally aligned along the length of the bars, becomes oriented orthogonally to the bars of the code see, U.S. Pat. No. 4,896,026, issued Jan. 23, 1990). The desired averaging, over the length of the bars and spaces, to minimize the effect of printing defects, is therefore not obtained throughout the working range of the scanner. Moreover, the beam becomes incapable of resolving fine (narrow or high resolution) bars and spaces. This situation can be alleviated by the use of an anamorphic optical system in which the beam is colliminated in the vertical direction (along the length of the bars of the code) and focused in the horizontal direction (see, U.S. Pat. No. 4,820,911, issued Apr. 11, 1989).
It is therefore desirable to provide a beam forming and shaping system wherein the optics provide an elongated beam which maintains its width to height aspect ratio over the entire range in front of the scanner (which may be a one to several feet long range) and is oriented with a long axis along the bars and spaces of the code and not transverse thereto. This objective has been accomplished in accordance with the invention through the use of optics which operate by diffraction and which utilize to advantage the diverging characteristics of the beam from a visible laser diode. The beam forming system of the invention has many advantages over traditional optics which are designed in accordance with geometric optic principles and which, in practice, as discussed above are subject to having the long axis of the beam flip so that the beam is no longer orientated with its long axis along the length of the bars and spaces in the code and are subject to decreased resolution and enhanced sensitivity to defects in the code.
Although there has been some statements in publications and patents that diffraction mechanisms are at work when a focused beam passes through an Aperture (see A. Erteza, "Active Autofocusing Using an Apertures Gaussian Beam", Applied Optics, Volume 15, No. 9, pages 2,095-2,096 (September 1976) and U.S. Pat. No. 4,808,804 issued Feb. 28, 1989), it has not been appreciated how to use a diffraction mechanism and diffractive optics to advantage in shaping a beam, particularly a diverging beam from a laser diode, and maintaining it in proper orientation and aspect ratio with respect to the bars of a bar code. In addition, an optical beam shaping system in accordance with the invention requires fewer components (physical aperture stops and their supporting mechanisms can be eliminated), and the complexities of anamorphic optics are avoided.
To this end, the invention provides a system (method and apparatus) for producing a pattern of generally monochromatic light having a predetermined configuration and orientation over a range of distances in which the symbol to be recognized, for example a bar code, may be located. A diffracting element is placed in the path of the beam which forms the light in the beam in a profile having a desired configuration and orientation, preferably an elliptical configuration in which the major axis of the beam is in a direction along the bars and spaces of the code, due to far field diffraction in the range where the code is to be resolved. Far field diffraction is defined as diffraction which results beyond the Fresnel distance from the source or the exit pupil of an optical system following the source. Preferably this exit pupil is provided by a lens of short focal length which brings the end of the range where far field diffraction effects occur in the vicinity of the window of the scanner through which the beam is projected or in the vicinity of the plane closest to the scanner in which it is desired to read a coded symbol. In the far field, the beam diverges at an angle which is approximately inversely proportional to the effective aperture of the beam. This effective aperture is preferably provided at the exit pupil, defined optically in a lens which focuses the beam from the laser. If the beam diverges, as it does from a laser diode, the effective aperture will have a long dimension and a short dimension which may be oriented with respect to the bars of the code. No physical or hard aperture is required. Through the use of the lens, the Fresnel distance due to the longest dimension (length) of the aperture (since the shortest Fresnel distance is proportional to the length of the aperture squared, the Fresnel distance for the short dimension occurs before the Fresnel distance for the long dimension) is typically located inside the scanner and ahead of the scanner's window through which the beam is projected towards the code. In long range scanning applications, the Fresnel distance for the longest dimension may be located at the closest desired reading plane, which may lie exterior to the scanner. The focal length and location of the lens is adjusted with respect to the location of the laser diode and the divergence of its beam so that the effective or phantom aperture within the lens (which is located at the principal plane of the lens where the lens starts to focus the beam) is such that the aspect ratio (length to width of the elliptical beam) remains generally constant over the range where the code is located for scanning. Accordingly, far field diffraction shapes the profile of the beam into a spot of width and length where the beam is incident on the code (in the plane of the code) so as to provide the desired shape without the need for anamorphic optics and using to advantage diffraction effects, to obtain high resolution scanning. The invention does not depend upon traditional geometric optics for shaping and focusing the beam wherein diffraction effects have produced undesirable results.
The optics of a scanner, particularly the laser and its beam projecting optics which produce the outgoing beam and the detector which receives the incoming beam have traditionally been located in alignment. In order to save space and reduce the size of the scanner head and particularly the scan module in the head, the photodetector and the laser have been mounted on a plate, usually a printed circuit board in offset relationship. Then the symmetry between the incoming and outgoing beams is lost and parallax is introduced. Parallax is the difference in apparent direction of an object as seen from two different points not on a straight line with the object. In a scanning system, this translates into a nonuniformity of return signal across the scan. If the change in signal across the scan is large, the dynamic range of the electronic circuitry may not be enough to compensate for the change in signal.
It has been found in accordance with the invention that the parallax problem can be solved, even though the photodetector and the laser are physically out of alignment with each other and are spaced apart on a single board on which they are mounted. Briefly described, the arrangement utilizes a fixed mirror from which the laser beam is reflected to an optical deflector, such as an oscillatory mirror, and is projected from the deflector over a scan path with traverses a center of scan as the beam scans back and forth across the code. The mirror also collects the return light and directs it to the photodetector. The oscillatory mirror, the laser and the photodetector are arranged with respect to the fixed mirror so that the outgoing beam (when it is in the center of the scan) and the return beam travel along paths which are in the same plane. Symmetry is therefore provided between the incoming and outgoing beams and parallax is eliminated.
It has traditionally been the practice to mount the optical assembly of a scanner to the case or housing in which the scanner is contained with shock mounts; the purpose being to isolate the optical assembly from shocks which could be transmitted to the assembly if the scanner was dropped onto the floor. The assembly of the scanner with shock mounts increases its cost due to the additional cost of the shock mounts and the additional labor to install the mounts. Moreover, resonances in the mechanical system can be introduced due to the compliance of the mounts which amplifies the shock forces.
It has been found in accordance with the invention that the optical assembly can be mounted on a single board which is held in the scanner housing in tracks which may be provided by channels formed around the inside surface of the housing. The board is inserted into the channel on one-half of the housing and then into an opposing channel in an opposite half the housing when the housing is assembled by bringing the housing halves together and fastening them together by attachment devices. It has been found that this mounting arrangement is more advantageous than shock mounts in that it avoids resonances introduced by the shock mounts. The shock forces which are applied to the board via the mounts has a frequency spectrum which includes frequencies at which the mechanical system of the board and the mount are resonant. Therefore shock mounting can give rise to destructive forces. Such forces do not occur in the mounting system provided by the invention wherein the board is held in the channels in the housing halves.
Operation of a scanner involves turning the laser on and off in response to trigger signals from a switch on the scanner or from an external terminal or control computer system to which one or more scanners are connected in a network. These control computers have facilities for decoding the bar code and also for turning the scanner on and off when a code is to be read. It is desirable that the scanner be universally applicable for use with various control or host computers which may provide commands or which provide data for programming the scanner. This data may be in different formats, for example signals of different level or polarity. Heretofore, bar code scanners have been designed for compatibility with only one type of computer or with one family of computers which utilizes the same code, protocol or data format, as regards level and polarities.
It is also necessary, particularly to comply with governmental standards concerning radiological health and safety, that the laser power be regulated to be maintained within the certain power and intensity levels. In addition, there are circumstances, such as operation in high temperature environments, where the current for operating the laser increases to a level that destroys (burns up) the laser. Laser diodes are particularly sensitive to the level of current which is used to drive them and can be destroyed if the current exceeds a safe level.
Another problems arises out of the variation and intensity of the scattered light which is returned from the code. Analog automatic gain control circuits have been used to vary the gain of the preamplifiers and amplifiers following the photodetector so that the digitizer which provides the analog bar code signal does not become overloaded on the one hand or does not receive signals of such low level that they cannot be tracked and converted with high accuracy.
A still further problem is in the area of controlling the beam so that it scans at essentially constant velocity across the code. Motor control circuits have been proposed for driving motors for oscillating a mirror so as to deflect and scan the beam (see for example U.S. Pat. No. 4,496,831 issued in September 1985, wherein opposing motor drive current are controlled in an analog fashion to one for electromagnetic bias and the other for driving force generation.
Still another problem arises when there are a number of closely spaced codes on a package or sheet which must be individually read. The scan wide enough to read any one code may be positioned so as to overlap adjacent codes thereby causing misreads or erroneous reads. In addition, a wide scan, particularly from a visible laser diode, spreads the intensity over a large distance so that it may be too dim to be observed for training the beam on the code of interest. In this connection it has been proposed to use a narrow scan and, when a few bars are read, to automatically increase the scan length (see U.S. Pat. No. 4,933,538 issued Jun. 12, 1990). If the scan length is automatically increased, it still can overlap an adjacent code which is not intended to be read.
All of these problems are resolved in accordance with the invention by a digital control system utilizing a computer. The computer generates digital gain control signals depending upon the intensity of the return light detected on a scan and adjusts the gain digitally so that on a subsequent scan, the signal level is optin:um. The digital gain control can be non-linear by changing the relationship between the value of the digital control signal and the intensity of the detected light in a way to more quickly bring the gain of the system to its proper value than would be the case with an analog automatic gain control system. The digital control control of gain is afforded by a digitally operative potentiometer in the amplifiers which amplify the photo detected bar code signal and which can be set to provide the required resistance corresponding to a desired level of amplifier gain.
Digital control, again utilizing a digital potentiometer in a control loop, regulates the current which drives the laser so that the digital control computer can control the laser power. The laser power can be regulated to a preset value dictated by governmental health and safety regulations on initialization utilizing a digital control signal corresponding to laser output power of desired level.
The digital control computer can also generate digital control signals for operating the motor which drives the deflector (the scanning mirror). These signals may be in the form of pulses which are pulse-width modulated (vary in duration) so as to control the instantaneous velocity as well as the frequency of oscillation. The control signals also control the scan length and may be varied in response to a manual actuator which produces a signal that depends upon the pressure exerted by an operator on a trigger or control lever of the scanner. When the trigger is released or has only light pressure applied thereto, the length of the scan can be set to be quite small or stationary. This small or short scan is quite bright and enables the beam to be located on the code by visual observation of the spot of light incident on the code. Then by increasing the pressure on the trigger or control lever, the digital control signal which drives the motor changes so as to increase the scan length either linearly or non-linearly in response to pressure. The length can be increased to be just sufficient to scan the particular code of interest and not overlap any adjacent codes.
Universal operation with various types of host or control computer systems regardless of their format can be obtained by programming the control computer in the scanner from the host so as to provide the bar code signals which are received by the host and the commands which are generated by the host and transmitted to the scanner in the desired polarity and level and with the desired protocol or format.
With the inventive system hereof, all of the control functions necessary or desirable from the scanner are obtained by way of digital control with a microcomputer which is mounted in the scanner.